The present invention relates generally to electrical power conversion systems and more particularly to active front end power converters with failure prevention using peak detection with decay. Power conversion systems supply power to a load by conversion of input power to output power of a different form. Active front end power converters include a switching rectifier stage that receives AC input power at a line frequency and converts the AC power to provide a DC bus voltage or DC link current to a load, which may be a subsequent power converter stage. In motor drives, for example, the DC power from the active front end is used by a subsequent inverter to create variable frequency AC output power to drive a motor. In wind energy systems, an active front end receives AC input power provided by a wind turbine generator, and converts this into DC power for use by a subsequent inverter stage that provides energy at a controlled phase and frequency to a power grid. In motor drives and other power converter applications, the IGBTs or other switching devices of the active front end can be damaged or degraded by excessive currents that may result from unbalanced phase voltages stemming from various faults or noise sources present in a power distribution system.
In the past, these problems have been addressed by oversizing the active rectifier switching devices to accommodate source imbalance situations or by operating the converter below rated output to prevent switching component damage or degradation. U.S. Pat. No. 7,355,865 to Royak et al., assigned to Rockwell Automation Technologies, Inc., provides control of second order harmonics in voltage imbalance situations using current regulation in the stationary reference frame, the entirety of which patent is hereby incorporated by reference herein. U.S. Pat. No. 8,009,450 to Royak et al., assigned to Rockwell Automation Technologies, Inc., describes compensating voltage command values for individual phases according to line voltage imbalances to provide balanced phase currents in the presence of unbalanced phase supply line voltages, the entirety of which patent is hereby incorporated by reference herein. Many power conversion systems also include shutdown circuitry for overcurrent protection, which is triggered by detecting a dangerous level of instantaneous peak current, sometimes referred to as an overcurrent level or IOC. In these systems, the trigger level is typically set to a value higher than rated current, such as about 220% of rated current. Certain power distribution fault conditions, however, may involve current levels sufficient to cause thermal stress to IGBTs even though the instantaneous current does not exceed the IOC level. For instance, the RMS value of one or more individual phase currents resulting from some faults in a Grid may be quite large and potentially cause thermal damage to an active rectifier switching device, while the corresponding instantaneous current values remain below the IOC level.
Other fault conditions can be caused by generation of current and voltage spikes on power lines that feed motor drives or other power conversion systems. For instance, welding equipment and other electrical noise sources may be connected to a power system, and thermal of overloading of active front end switching devices may result even if there is no fault in a Grid or resulting phase voltage unbalance condition. Consequently, a need remains for improved power conversion systems and techniques for protecting active front end switching devices against noise and voltage or current imbalance fault conditions.